Here is a story in today's Wired News about Henry Niman and his ideas about viral evolution in the Avian Flu (H5N1). While the text of the story is a bit unclear about the difference between recombination and reassortment, one of the associated images is quite nice. This is yet another take on the specific mechanisms of viral evolution. The figure defines reassortment as the emergence of a new strain via the replacement of whole genes from another (related) virus, and defines recombination as the insertion of fragments of genes into a new viral strain from another genome, potentially from the host.

Ignoring what labels are used, it seems the important point is that there may be two mechanisms for introduction of new sequences into an influenza viral genome; 1) inclusion of whole genes into a segmented genome or 2) insertion of gene fragments from another strain or species within a given viral gene.

Niman seems to think that not only is there evidence that the current H5N1 strain is evolving via the second mechanism, but that this is also the origin of the Spanish Flu (see my post "The Spanish Flu Story"), despite the fact that there appears to be a historically low occurrence of homologous recombination in negative sense RNA viruses (see my post "A Confluence of Concerns").